The invention relates to a luminescent material comprising zinc silicate activated with Mn2+. The invention also relates to a luminescent screen, a plasma panel display and a discharge lamp.
A luminescent material comprising zinc silicate activated with Mn2+ is known for instance from EP 0393754, to which U.S. Pat. No. 5,041,758 corresponds. Such a luminescent material is widely used in fluorescent lamps and in plasma display panels to convert exciting radiation into visible green light. In fluorescent lamps the luminescent material is mainly excited by radiation with a wavelength of 254 nm, whereas in plasma display panels the excitation takes place under the influence of VUV radiation (10 nm-200 nm). Although the efficiency of the conversion depends on the wavelength of the exciting radiation, it can be concluded that in both applications the overall efficiency of the conversion of the exciting radiation into visible green light is relatively poor. Furthermore the decay time of the luminescent material is too long for displaying e.g. motion pictures on a plasma display panel.
The invention aims to provide a luminescent material comprising zinc silicate activated with Mn2+, that converts exciting radiation within a wide range of wavelengths relatively efficiently into visible green light and has a relatively short decay time under VUV excitation.
In accordance with the invention, zinc silicate is doped with one or more dopant ions chosen from the group formed by Gd3+, Eu2+, Co2+, Ce3+, Pr3+, Nd3+, Sm3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, and Yb3+.
It has been found that a luminescent material according to the invention has a relatively high absorption for exciting radiation within a wide range of wavelengths. Furthermore the quantum yield of luminescent materials is also relatively high. As a result, luminescent materials according to the invention show a relatively high overall efficiency of the conversion of exciting radiation into visible green light for exciting radiation within a wide range of wavelengths. It has further been found that a luminescent material according to the invention has a relatively short decay time under VUV excitation.
In case the zinc silicate is doped with a trivalent ion, the luminescent material preferably further comprises one or more ions chosen from the group formed by Li1+, Na1+ and K1+. By choosing the amount of monovalent ions substantially equal to the amount of trivalent ions, it is realized that the luminescent material has no net electrical charge.
Good results have been found for luminescent materials wherein between 0.1% and 15% of the Zn2+ ions of zinc silicate have been replaced by Mn2+ ions.
Similarly good results have been obtained for luminescent materials wherein between 0.05% and 5% of the Zn2+ ions of zinc silicate have been replaced by dopant ions.
Because of the properties described hereabove, a luminescent material according to the invention is very suitable to be used in a luminescent screen that is part of a plasma display panel or of a discharge lamp. More in particular, good results have been obtained for discharge lamps comprising a gastight lamp vessel containing mercury and gas.